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Posted on November 6th, 2013

Outside the Box – Shade Trees

When I moved from Colorado to Texas, I knew there would be things I missed (oh, you’ll never know), but one thing I didn’t think about much, strangely, was the lack of trees in my new place of residence. By nature I’m an outside guy; I love to go hiking and play in the woods (not so easy down here), but I also love to sit outside and read, listen to music or sometimes, just think. And in Colorado, I would do that with the help of some beautiful shade trees, such as the English oak, hackberry or American elm.

Ten-year running average temperatures in Los Angeles, California.

Down here in Texas, trees are few and far between. The ongoing droughts have played a significant role in this, but I do my best to keep the trees around my house watered and healthy to ensure a place for me to sit and read (it is 90 degrees and 65% humidity right now), and also to help keep my cooling costs down. You see, in East Central Texas, air conditioning units run all day for close to 7 months a year. So to help fight the sting of that monthly utility bill, it is nice to have efficient insulation, and some big ol’ trees to block the mean ol’ sun.

Researchers at the Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California have been studying the impact of shade trees for some time now. They note that modern urban areas typically have darker surfaces and less surrounding vegetation. This can alter the local climate by heating the air to create warmed urban islands. Have you ever noticed on your drive from the city to the country that sometimes it feels a little cooler? These scientists say that the dark surfaces and typical urban structures like houses and buildings can effectively raise the temperature a full 2.5 degrees as opposed to rural areas.

The researchers also looked at the effects of “cool roofs” on surface, retention and reflective temperatures. Over roofs with low-albedo surfaces (i.e. highly absorptive of the sun’s rays), the difference between surface and ambient temperatures may be up to 50 degrees Celsius, thus causing the surrounding air temperature to increase. Thanks in part to vegetation cover or lighter colored surfaces, roofs with high-albedo surfaces (i.e. highly reflective cool roofs) had a 10 degrees Celsius reduction in temperature. In the simulations generated by their model, they predicted the potential reduction in the ozone effect. They equated a large-scale transition to cool roofs to have the potential to reduce heat island effects similar to replacing all gasoline vehicles with electric cars – whoa!

The LBNL methodology to analyze the impact of shade trees, cool roofs and cool pavements.

Applying a high-albedo coating to roofs was estimated to have a seasonal savings of 20%, with peak demand reductions of about 25% of pre-coated use. In a controlled experiment of nine homes in Florida, air-conditioning energy use was reduced between 10-43% after application of the coating. In a consideration of the urbanized area in the L.A. basin, it is estimated that roughly 50% of the area is covered by roofs and roads. If an area-wide application of high-albedo coating were to be applied, it would cool the affected area by 2 degrees Celsius during the hottest periods of August, thus saving an estimated $20+ million in energy costs.

Another consideration of the study, as alluded to in the introduction here, is the benefit of shade trees, and their ability to intercept sunlight before it warms a building. Aside from keeping buildings cooler by blocking the sun and thus lowering air-conditioning costs, trees also lower ambient air temperature which increases air quality. Other intangible effects of shade trees are increased property values, and decreased rain water run-off which can be beneficial in the protection against floods (something very near and dear to Boulder, Colorado right now).

The cherry on top here is that lighter colored surfaces increase the life expectancy of both roofs and pavement, and the planting of trees helps to reduce smog, which can in turn lower the air quality index ratings of larger cities during peak hot periods. So take a look at the heating and cooling expenses of your home; and then consider the minimal changes you can for big savings, and a better life.

Justin Harmon
Staff Writer

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